The uses available from polymer based composite materials has been considerably broadened by the introduction of thermoplastic (TP) matrices. Thermoplastic polymers, in particular semi-crystalline ones, provide a number of significant advantages over their thermosetting counterparts. They exhibit better fraction toughness, there is no need for chemical reaction during processing and therefore no toxicity during processing and application, and they are recyclable.
On the other hand, the high viscosity of the molten thermoplastic resin imposes many limitations on the manufacturing process with these materials. These are usually related to the need of incorporating the highly viscous polymer into the structure made of continuous reinforcing fibers. Use of commingled yarn is one of the most promising approaches for fast production of TP based composites. Commingling of thermoplastic and reinforcement fibers gives a good initial matrix-reinforcement distribution in a non-molten state before processing. This represents a kind of dry impregnation, which circumvents some of the difficulties associated with the high viscosity of the molten polymer.
Fabric woven from commingled yarn can be processed by application of heat and pressure into composite materials in a manner analogous to that of thermoset based prepregs. The relatively uniform commingling of the reinforcing matrix fibers minimizes the distance the molten thermoplastic is required to flow to fill the inter fiber spaces and this allows the material to be consolidated using reasonable pressure even at high fiber content of glass fibers.
Today, compression molding is the most important manufacturing process used for commingled fabrics. When fabric is heated above the melting point of the resin, the thermoplastic fibers melt and flow around the glass fibers under applied pressure and fill the space between them. The initially separated yarn, now consisting of reinforcing fiber bundles and the molten polymer move towards one another, and the polymer flows out of the bundles to fill the free space between them. As the pressure increases the smaller inter-fiber voids are filled leading to a, hopefully, full consolidation (i.e. void free material). When consolidation is finished, the composite material has to be cooled under pressure, down to a temperature where the resin is solid enough to avoid unwanted deformation after demolding. These are the problems encountered in high volume fabrication of TP composite that necessitates use of automated, well controlled, and expensive fabrication process.